# Python | Reverse an array upto a given position

• Difficulty Level : Medium
• Last Updated : 21 Nov, 2018

Given an array arr[] and a position in array, k. Write a function name reverse (a[], k) such that it reverses subarray arr[0..k-1]. Extra space used should be O(1) and time complexity should be O(k).

Examples:

```Input: arr[] = {1, 2, 3, 4, 5, 6}
k = 4

Output:  arr[] = {4, 3, 2, 1, 5, 6}
```

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This problem has existing solution please refer Reverse an array upto a given position link. We will solve this problem quickly in Python.

 `# Program to Reverse an array ``# upto a given position `` ` `def` `reverseArrayUptoK(``input``, k):`` ` `    ``# reverse list starting from k-1 position ``    ``# and split remaining list after k``    ``# concat both parts and print``    ``# input[k-1::-1] --> generate list starting``    ``# from k-1 position element till first ``    ``# element in reverse order``    ``print` `(``input``[k``-``1``::``-``1``] ``+` `input``[k:])`` ` `# Driver program``if` `__name__ ``=``=` `"__main__"``:``    ``input` `=` `[``1``, ``2``, ``3``, ``4``, ``5``, ``6``]``    ``k ``=` `4``    ``reverseArrayUptoK(``input``, k)`

Output:

```[4, 3, 2, 1, 5, 6]
```
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